The present invention relates to visualization of a transmission status in a digital transmission system and more particular, to a technique for retransmitting display data in relay transmission.
For radio transmission of video and voice signals, an analog transmission system has been used until several years back. In these years, however, a digital transmission system based on QAM (Quadrature Amplitude Modulation), OFDM (Orthogonal Frequency Division Multiplexing) or the like is used.
In the latter case, data to be transmitted is data about a transmission signal such as a TS (transport stream) obtained by compressing a video or voice signal in MPEG processing. In this case, however, there is generally used a digital transmission system of a type wherein a digitally-modulated transmission signal is mapped to two-dimensional data and transmitted from a transmission side, and a reception side identifies the two-dimensional data and reproduces it into the original transmission signal.
In the case of the aforementioned analog system, the SN (or S/N) ratio of the video or voice signal varies with the level of a received electric field. For this reason, in mobile transmission of, e.g., a marathon relay broadcast, a relayed video tends to be affected by much noise or disturbance, thus resulting in a low quality of image.
When the digital transmission system is employed, however, digitalized information is transmitted and thus error correction can be applied thereto. As a result, even in such a transmission environment in which the level of a received electric field varies, an identical quality of images can be relayed so long as the field level is within an error correctable range.
In the digital transmission system, on the other hand, if the field level is lower than a predetermined limit value, then the error correcting function cannot work. In this case, transmission of the video signal becomes abruptly impossible, but a lower limit value at the then field level can be known to a certain extent according to the signal status of a reception side in the identifying/deciding operation.
For example, in the case of a 64 QAM system having a transmission rate of 60 Mbps which is often used, a carrier-to-noise (C/N) ratio has a minimum of about 27 dB and thus the lower limit value of the received field level is about −70 dBm. Accordingly, the field level is required to be higher than the lower limit value for video transmission.
In the case of a 16QAM2 system having a transmission rate as relatively small as 35 Mbps, the C/N ratio has a minimum of about 18 dB and thus the lower limit value of the received field level is about −80 dBm, resulting in that the video can be transmitted so long as the field level is higher than the lower limit value.
As a digital transmission system to which such a data correcting function is applied, there is already known a system in which the lower limit value of level of a received electric field is recognized based on a transmission status or a synchronous reproduction status, as disclosed in JP-A-6-326735 or JP-A-2002-223459.
An example of such a digital transmission system will next be explained by referring to FIG. 15. The drawing is an exemplary prior art when a digital transmission system having a data correcting function applied thereto is used to relay a video from a site or point A to a site or point B. In this case, for example, the point A corresponds to an photographing site where a relay car is capturing an image or images of a running marathon athlete or athletes, the point B corresponds to a broadcast station, and broadcast transmission between the points A and B is carried out wirelessly using radio waves, e.g., in a microwave band.
At the point A, a video signal taken by a television camera (not shown) is input to an MPEG encoder 1, where the input signal is converted to compressed data TS, and then the data TS is input to a mapper 2 for determining a modulation mode, where the input compressed data TS is converted to two-dimensional data Dm.
The data Dm is modulated by a modulator (MOD) 3 to an intermediate frequency signal Dmod in a 130 MHz band, and the signal is supplied to a high frequency (microwave) transmitter 4. In the transmitter, the intermediate frequency signal Dmd is converted to a signal having frequencies in the microwave band, power-amplified, sent to an antenna 5, and then transmitted from the antenna 5 as a microwave W1 to an antenna 6 located at the point B.
The microwave W1 transmitted to the antenna 6 at the point B is received at the point B as a microwave signal and then input to a high frequency receiver 7. In the receiver 7, the received weak signal is amplified and converted from the signal in the microwave band to and an intermediate frequency signal Ddem in the 130 MHz band.
The intermediate frequency signal Ddem is then input to a demodulator (DEM) 8, where the intermediate frequency signal is subjected to timing and frequency reproducing operations, that is, is demodulated to two-dimensional data Dd having an in-phase component I and a quadrature component Q. The two-dimensional data Dd is restored by an identification decider 9 to reproduced compressed data TSr, and then input to an MPEG decoder 10, where the data TSr is expanded to a video signal.
At this time, since the level of the received electric field has a lower limit as mentioned above, it is required to recognize the good or bad status (quality) of the transmission or synchronous reproduction. To this end, the system is arranged so that the two-dimensional data Dd output from the DEM 8 is supplied also to a display unit 11 as an X-Y input of an oscilloscope provided in the display unit, whereby a constellation can be observed on the display though short in its displayable time.
In this case, the constellation displayed on the oscilloscope of the display unit 11 is as shown in FIGS. 16A and 16B. FIG. 16A is a display when the transmission or synchronous reproduction status, in which case mapped points are combined into a small group. However, when the status becomes bad, the mapped points becomes large and blurred as shown in FIG. 16B.
Thus, by observing the constellation displayed on the oscilloscope, the user can recognize the transmission or synchronous reproduction status and can determine whether or not the level of electric field reaches its transmittable range.
For this reason, the user can anticipate a danger of interruption in the video transmission and can previously take measures against it, for example, can change over to another program previously prepared.
In the aforementioned prior art, however, no consideration is paid to a situation where at least one relay point is provided in the transmission line of relay data leading from the point A via the point B further to the point C. Thus when the broadcast station wants to monitor the transmission status or statues of specific one or ones of such relay points, the prior art has a problem that the station cannot recognize the reception status of a relay point on the way.
For example, when the point A is located away from the broadcast station or when such an object as to cause a trouble in radio wave propagation exists on the way, it is required to provide a relay point on the way to the broadcast station. In this case, as shown in FIG. 17, the relay point corresponds to the point B which is located, e.g., on a low hill, and the point C corresponds to the broadcast station.
In this case, FIG. 17 corresponds to the system arrangement of FIG. 15, but is different from FIG. 15 in that a retransmitter 12 and a transmitting antenna 13 are newly added as a relay point, and a receiving antenna 14 and a receiver 15 are newly added to a reception side such as a broadcast station.
And the retransmitter 12 and the transmitting antenna 13 are provided in the point B, the receiving antenna 14 and the receiver 15 are provided in the point C, and transmission between the points B and C uses, e.g., a wave W2 in the microwave band. Thus, the transmission system of FIG. 17 is a 2-stage relay transmission type.
In this case, the retransmitter 12 in the relay point receives the reproduced compressed data TSr issued from the identification decider 9, and transmits the data carried on the microwave W2 from the antenna 13 to be received by the antenna 14 located at the point C.
The antenna 14 receives the microwave W2 and sends it to the receiver 15, where the microwave W2 is to be reproduced into a video signal. In this case, the broadcast station as the point C cannot monitor the reception status of the point B.